Valve arrangement,especially for use in anaesthetic gas supply systems

ABSTRACT

A blow-off valve which vents excess gas to the atmosphere when the pressure in a gas circulation system, such as an anaesthetic supply system, exceeds a certain value, as occurs when the patient breathes out, is improved by providing a system to close the valve when the pressure exceeds a still higher value, as occurs when gas is forced into the patient&#39;&#39;s lungs. A movable member is exposed to the gas pressure in the system. A force transfer means connects the movable member in the valve and closes the valve when this higher pressure is reached.

United States Patent Hesse, deceased et al.

[ 51 Oct. 24, 1972 [54] VALVE ARRANGEMENT, ESPECIALLY FOR USE INANAESTHETIC GAS SUPPLY SYSTEMS {72] Inventors: Holger Hesse, deceased,late of Skovtoftebakken l9, Virum, Copenhagen, Denmark by Ruth LeaHesse, heiress and executrix', Henning M. Ruben, Skodsborg Strandveg260, Copenhagen, Denmark [22] Filed: March 3, 1969 [21] App]. N0.:804,741

[30] Foreign Application Priority Data March 4, 1968 Great Britain..10,419/68 [52] US. Cl. ..l37/494, 128/1456, 251/65,

[51] Int. Cl. ..A62b 7/00, A61m 17/00 [58] Field of Search..137/494, 63R; 128/1457, 202,

[56] References Cited UNITED STATES PATENTS 2,222,710 11/1940Goldschmidt ..251/23l X 3,200,818 8/1965 Johannisson 128/202 3,351,09211/1967 lngerfield et al. ..251/65 X 3,400,908 9/1968 Bauer ..251/61 X3,473,529 10/1969 Wallace ..128/145.7 3,503,393 3/1970 Manley 128/1456Primary Examiner-M. Cary Nelson Assistant Examiner-Robert J. MillerAttorneySpencer & Kaye 7] ABSTRACT A blow-ofi valve which vents excessgas to the atmosphere when the pressure in a gas circulation system,such as an anaesthetic supply system, exceeds a certain value, as occurswhen the patient breathes out, is improved by providing a system toclose the valve when the pressure exceeds a still higher value, asoccurs when gas is forced into the patient's lungs. A movable member isexposed to the gas pressure in the system. A force transfer meansconnects the movable member in the valve and closes the valve when thishigher pressure is reached.

30 Claims, 8 Drawing Figures PATENTEUnm 24 1912 SNEEI 2 IF 7 PATENTED3.700 000 saw u or 7 PATENTED B I 3 700 000 SHEET 5 BF 7 FIQA PATENTED24 I97? 3, 700 O00 sum 5 BF 7 VALVE ARRANGEMENT, ESPECIALLY FOR USE INANAESTHETIC GAS SUPPLY SYSTEMS The present invention relates to valvearrangements adapted for use in connection with such gas circulationsystems as, for example, anaesthetic gas supply systems i.e. systems inwhich anaesthetic gas and/or oxygen are introduced into the lungs of aperson to be treated. More specifically, bot not exclusively theinvention refers to valve arrangements for intermittently opening andclosing a connection between a so-called semiclosed anaesthetic gassupply system and the atmosphere.

During narcosis it is in many cases a factor of greatest importance tosupport natural breathing or to administer artificial respiration, whichcan be performed with the aid of an anaesthetic gas supply system of thetype shown in FIG. 1 of the accompanying drawings. Such a systemcomprises a semi-closed circuit 1 continuously supplied with gas from agas source 2. The system also comprises an expandable bag or bladder 3which is filled with gas from the source 2 due to the fact that the gaspressure in the system is slightly higher than atmospheric pressure.Moreover the system is commonly provided with a connection 4 leading tothe respiratory system of the patient, with directional valves such asS, 6 forcing the gas the flow through the system in the correctdirection, and an absorber 7 removing pollutions such as carbon dioxidefrom the system. The gas is forced into the lungs of the patient bycompression of the bag or bladder 3 causing a state of overpressure tobe developed in the system. When the bag is subsequently released andthe pressure again drops to substantially atmospheric pressure or belowthe positive pressure then prevailing in the lungs will force the gasback into the anaesthetic gas supply system. As during the completeprocess oxygen and/or anaesthetic gas will be continuously supplied andas the amount of air expired is substantially of the same order ofmagnitude as the amount inhaled it will obviously be necessary toprovide a valve connection between the gas'system l and the atmosphere,thereby enabling exccss of gas to be discharged from the system.

The valve arrangements in conventional semi-closed rebreathing systemsof the above type usually operate in such a way that the valve disk ofablow-off valve 8 is lifted from its seat whenever the positive pressurein the system exceeds a predetermined value of, for example, about 0.5cm H due to spontaneous exhalation or excess supply of gas whereby suchexcess gas is discharged from the system. When the pressure prevailingin the system has a value lower than that mentioned above the valve diskdue to its normal bias, for example by means of a spring or the actionof gravity, will rest against its seat whereby the communication betweenthe re-breathing system and the atmosphere will be interrupted.

The so-called controlled ventilation of the patient, i.e. the measure offorcefully introducing gas into the patients lungs by compression of thebladder or bag, requires that a considerably higher positive pressure bebuilt up in the system, the order of magnitude of this pressure being,for example 30 cm 11,0. In many prior art devices the positive pressurenecessary for ensuring an adequate controlled ventilation is obtained bymaking the gas discharge capacity of the blow-off valve, on the one handsufficiently low to allow said positive pressure to build up in thesystem during compression of the bag and, on the other hand,sufficiently large to allow the excess gases and/or the exhalation gasesto blow-off" to the atmosphere without causing an unduly high pressureto appear in the system.

A considerable drawback of such valve arrangements may be ascribed tothe fact that no direct conclusions can be drawn from the degree ofcompression of the bag or bladder concerning the amount of gas actuallyintroduced into the lungs. Accordingly, it is common practice toevaluate this factor by considering the expansion of the thorax duringthe inhalation phase, such evaluation however being of a qualitativerather than quantitative character.

Another drawback of said value arrangements is that a considerablevolume of gas is lost during each compression of the bag this drawbackbeing particularly pronounced when the system is continuously suppliedwith an expensive anaesthetic gas.

It has been proposed to solve this problem by increasing the resistanceagainst opening of the blow-off valve, i.e. the closing bias of thevalve disk, by manual operations. However, this involves the seriousdrawback that due to an increase of the pre-loading of the valve diskthe blow-off valve will open during the expiratory phase only at anunduly high overpressure. Hereby the blood circulation of the patientmay be adversely influenced because the generally excessive pressureprevailing in the lungs caused thereby will oppose the inflow of bloodto the heart. These conditions will be aggravated in proportion to theamount of gas supplied to the narcosis system.

As breathing and blood circulation are vital functions which whenchanged during acute situations may involve danger of life for thepatient it is an advantage or even a necessity so to operate ananaesthetic gas supply system that the expiratory phase may take placeduring normal pressure conditions while still enabling the system to beoperated in such a way that any compression of the bag causes an amountof gas substantially corresponding to the amount expelled from the bagto be introduced into the lungs.

It is a purpose of the invention to solve the above problems referringto opening and closing the commu nication between the rebreathing systemand the atmosphere in such a way that this communication is held openfor discharging excess gas and expiration air, the communication beingclosed when the bag or bladder is compressed i.e. when the gas is to beforced into the lungs of the patient, the communication being openedagain when the compression of the bag is discontinued. Under theseconditions the pressure in the anaesthetic gas supply system during theexpiration phase is limited to a minimum value corresponding to thevalue required in order to guarantee the expansion of the bag or bladderdue to the continuous supply of gas whereby the bags character as a gassupply reservoir will be maintained.

In accordance with the invention this problem is solved by using, on theone hand, a blow-off valve such as a disk, poppet or membrane valve of aconventional type keeping the communication between the rebreathingsystem and the atmosphere closed up to a relatively low pressure butadapted to open this communication when there is an excess amount of gaspresent and when expiration takes place and, on the other hand, a deviceresponsive to the pressure prevailing in the system and adapted to actupon the valve disk of the blow-off valve to close this valve again uponcompression of the bag or bladder, i.e. under conditions of anoverpressure in the system substantially greater than that at which thebiasing force acting on the valve disk is overcome. This result isobtained according to the invention by causing at some place in thesystem the pressure prevailing therein to act on a movable member whichin turn develops a pressure-dependent force which is transferred to thevalve disk of the blow-off valve and acts to close the valve when thebag is compressed. It is then necessary, in order that the valve shallbe closed upon compression of the bag, that the pressure force appliedto the valve disk from the movable member during the compression isgreater than the difference between the pressure force acting on thevalve disk due to the pressure prevailing in the system and tending toopen the valve and the biasing force holding the valve closed during lowoverpressures in the system. If no particular arrangements are madesubstantially the same pressure will act on both the movable member andthe valve disk, and in this case the above presupposition means thateither the pressure within the system will have to be caused to act onthe movable member over greater area than the area of the valve disk orthat a suitable transfer ratio must be applied in the force transferfrom the movable member to the valve disk. Of course these two means forobtaining an overwhelming closing force acting on the blowoff valve uponcompression of the bag may be combined.

A further condition for the proper functioning of the valve arrangementaccording to the invention is that the force developed by the movablemember must not be transferred fully to the valve disk at lowoverpressures prevailing in the system because this would mean that theblow off valve would not open for discharging excess gas after fillingof the bag or during the expiration phase. The system would in this caseoperate as a completely closed system. The desirable way of operation isobtained in accordance with the invention either by providing meansexerting a certain, preferably constant and pressure independent forceopposing the closing force developed by the movable member, or byproviding means making said closing force act upon the valve disk of theblow-off valve only when this force has attained a certain value, iewhen the pressure in the system has reached a certain value higher thanthat at which the biasing force of the blow-off valve is overcome.

Thus, the primary object of the invention is to provide a valvearrangement for use in such gas circulation systems as anaesthetic gassupply systems, in which a connection between the system and theatmosphere is to be opened or closed depending on the pressureprevailing in the system, this arrangement comprising a blow-off valveinterconnecting the system and the atmosphere and provided with biasingmeans tending to hold the valve closed at small overpressures in thesystem but permitting the valve to be open at slightly higheroverpressures caused by supply of excess gas and during expiration, anda movable member connected to the system and adapted to be acted-upon bythe pressure prevailing in the system, said movable member developing aforce generated by this pressure and transferred via a force transfersystem to the valve disk to close the blow-off valve when the bag iscompressed and again to open or permit opening of the valve when thecompression is discontinued.

This and other objects of the invention will appear from the followingdescription of preferred embodiments of the invention with reference tothe accompanying drawings, in which:

FIG. 1, which has already been described illustrates a conventionalsemi-closed re-breathing system;

FIG. 2a and 2b illustrate a preferred embodiment of the valvearrangement according to the invention during different pressureconditions;

FIG. 3 shows another embodiment of the movable member;

FIG. 4 shows another embodiment of the force transfer system;

FIG. 5 and 6 illustrate another embodiment of the force transfer system;and

FIG. 7 shows still another embodiment of the valve arrangement accordingto the invention.

FIGS. 2a and 2!: represent a preferred embodiment of the valvearrangement according to the invention, FIG. 2a illustrating theinsufflation phase with compression of the bladder and FIG. 2billustrating the exhalation phase with release of the bladder. Accordingto this embodiment the valve arrangement according to the invention ismade very compact and comprises a housing 15 provided with an inletconnection 16, an outlet connection 17, and a bladder connection 18. Agas conduit 19 which interconnects the inlet connection 16 and theoutlet connection 17 is continuously supplied with oxygen and/oranaesthetic gas from a gas source 2. Said conduit 19 is also providedwith a branch conduit leading to the patient and an absorber 7eliminating pollutions such as charbon dioxide from the gas circulatingin the conduit 19. Said conduit also contains one-way valves such as 5forcing the gas to flow only in the correct direction in the gas conduit19.

The housing 15 is provided with a blow-off valve 8 putting the interiorof the housing in communication with the atmosphere, said valvepreferably being made up of a valve disk 14 and a corresponding valveseat 20. The valve disk 14 is subjected to a biasing force obtained by,for example, a spring pressing the valve disk against its seat and/or bythe gravity force acting on the valve disk. Said biasing force isadjusted such that the blow-off valve opens as soon as the positivepressure in the system attains a relatively low value of, for example,0.5 cm H 0, i.e. the pressure required for filling the bladder with gasafter compression.

According to the invention the housing 15 is also provided with aconnection to a movable member 9 which is shaped as a bellows in theembodiment shown in FIGS. 20 and 2b. The bellows 9 expands and contractsdepending on the pressure variations in the system. The pressuredependent force developed by the bellows upon expansion thereof istransferred to the valve disk 14 via a force transfer system in order toexert a closing force on the blow-off valve 8. The force transfer systemmay be designed in many different ways, a preferred embodiment beingshown in FIGS. 2a and 2b. According to this embodiment the bellows 9 iscaused to act upon the end of a lever 13, the other end of which isadapted to come into contact with the valve disk 14 in a direction toclose the valve.

The lever 13 is double-armed and rotatably fixed to a support structure26 which is secured to the valve housing 15. In order to obtain a goodforce transfer from the bellows 9 to the force arm 25 of the lever, theend of said arm is preferably formed as a thin plate, the surface ofwhich corresponds to the upper surface of the bellows 9. When utilizingthis embodiment of the force transfer system it is possible mutually toadapt the surface of the movable member 9 exposed to the pressureprevailing in the system, the corresponding surface of the valve disk 14and the length ratio between the force arm 15 an the load arm 24 of thelever in such a way that the pressure dependent force transferred fromthe bellows 9 and acting to close the blow-off valve will be greaterthan the pressure dependent force acting on the valve disk 14 andtending to open the blow-off valve.

The valve arrangement shown in FIGS. 2a and 2b also illustrates apreferable solution of the problem to prevent blocking of the blow-offvalve during the expiration phase, i.e. when relatively low overpressureexists in the system. According to this solution there is provided amagnet 21 adapted to attract an iron part 22 comprised by the forcetransfer system. This iron part 22 is positioned in such a way inrelation to the magnet 21 that the force of attraction of the magnet 21counteracts the closing force produced by the movable member 9 such thatthe load arm 24 does not close the blow-off valve 8 until the attractionforce of the magnet has been overcome. As the force of attractionbetween the magnet and the iron part decreases rapidly with the distancetherebetween there will be a very rapid and effective locking action assoon as the closing force from the movable member overcomes the saidattraction force. The value of the force of attraction can be controlledby means enabling the magnet 21 to be shifted in relation to the ironpart 22 belonging to the force transfer system. In the embodiment shownin FIGS. 2a and 2b the magnet 21 is disposed above the load arm 24 ofthe lever 13 comprising the iron part 22, but it is, of course, alsopossible to dispose the magnet below the force arm 25, which is in thiscase provided with the iron part. A simple way of varying the attractionforce between the iron part 22 and the magnet 21, which is preferablysupported by a carrier means 27 secured to the housing 15, is to providethe magnet with a screw thread for engagement with a corresponding nutthread in the carrier means 27, the attraction force then being variedby screwing the magnet more or less into the nut thread of the carriermeans, which may also be provided with a scale cooperating with a markon the magnet and indicating for each setting position, for example, theflow velocity or overpressure at which the blow-off will be blocked.

Another manner of preventing blocking of the blowoff valve at relativelysmall overpressures or flow velocities is to design the force transfersystem such that it will not come into action until the pressure in thesystem has reached a certain value. Such an arrangement involves theadvantage that the valve will not be acted-upon by a successivelyincreasing closing force but rather by a closing force which uponactivation of the force transfer system suddenly rises from zero to arelatively high value. Thus, the valve closing action will become moreeffective. If a bellows is used as the movable member the arrangementmay, for example, be suchthat the bellows actuates the force transfersystem only when a certain degree of extension of the bellows has beenreached. In the construction involving a piston movable in a cylinder acorresponding arrangement may consist in causing the piston to releasethe force transfer system only after a certain movement of the piston inthe cylinder.

The closing action of the valve arrangement according to the inventioncan be made more effective by providing a check valve such as a disk orflap valve in the system between the position at which the blow-offvalve is provided and the position at which the movable member isattached to the system. In this way the gas stream in the system may besuitably controlled to obtain, on the one hand, a more effective closingof the blow-off valve and, on the other hand, a delayed closing actionin response to pressure variations during the expiration phase.

As shown in FIGS. 20 and 2b the check valve 28 is preferably placed inthe proximity of the movable member 9 or the branch conduit leading tothe movable member, and the check valve 28 is directed so as to preventthe gas flow generated by compression of the bag from following theshortest way in the circuit towards the blow-off valve 8. If the movablemember 9 is positioned close to the bag 3 and the blow-off valve 8closely adjacent to the movable member while at the same time causingthe check valve 28 to prevent gas from flowing the shortest way betweenthe movable member 9 and the blow-off valve 8, the pressure surge due tocompression of the bag 3 will actuate the movable member 9 practicallywithout delay whereas the valve disk 14 of the blow-off valve 8 will beacted-upon only after propagation of the pressure surge throughpractically the whole system.

In this manner the pressure increase caused by compression of the bagwill immediately act on the movable member 9 developing apressure-dependent closing force, but said pressure increase will notreach the valve disk of the blow-off valve but after a certain period oftime. Consequently, the pressure acting on the movable member 9 willduring the compression of the bag 3 be higher than the pressure actingon the valve disk 14 and tending to open the blow-off valve 8.

On the other hand, pressure increases due to exhalation or excess supplyof oxygene or anaesthetic gas will first act on the valve disk 14 andonly thereafter on the movable member 9. On such occations when it isnot desired that the blow-off valve be closed the pressure acting on themovable member 9 will be equal to or rather somewhat lower than thepressure acting on the valve disk 14, thereby counteracting blocking ofthe blow-off valve.

The propagation of the pressure increase caused by compression of thebag can be further delayed by introducing a throttling means in thesystem. A very simple and efiicient embodiment of such throttling meansis shown in FIG. 2a. According to this embodiment the throttling meansconsists of a disk 29 having a central opening 30 of reducedcross-section. The disk 29 shown i FIG. 2a is disposed between the inletend of the housing 15 and the blow-off valve 8, but it is also possibleto insert the throttling means at some other place in the systemdownstream of the bladder and upstream of the blow-off valve. Any otherconventional throttling means such as different types of jets or nozzlescan be utilized in place of the disk 29 in order to reduce the openingforce acting on the valve disk of the blow-off valve during compressionof the bag and during exhalation.

Another suitable embodiment of the movable member is shown in FIG. 3. Inthis case the movable member has the shape of a piston which is slidablein a cylinder 1 l which is in communication with the main conduit of thesemi-closed re-breathing system, the displacement of the piston 10 beingdependent on the pressure in said system, which acts on one surface ofthe piston 10. The piston may be exposed to the action of a spring 12counteracting the force for which the piston is exposed due tooverpressure in the system and thus permitting transfer of the latterforce to the valve disk only when the force has become greater than thespring force, i.e. upon the precence of a relatively high overpressurein the system caused by compression of the bag or bladder 3. The piston12 may be mechanically connected to one end of a lever 13, the other endof which is adapted to come into contact with the valve disk 14 of theblow-off valve or to actuate said disk via suitable transmission membersas has already been described with reference to FIGS. 2a and 2b. Thespring 12 can also be replaced by the magnet arrangement describedabove.

In another embodiment of the invention which is illustrated in FIG. 4the force transfer system 23 is provided with an adjustablecounterweight 31 which is so arranged that the force transferred to thevalve disk 14 via the force transfer system 23 will not become largeenough to close the blow-off valve 8 until the overpressure in thesystem has reached a certain value at which the pressure force exertedon the movable member, for example the bellows 9 or the piston 10, isgreater than the force exerted by the counterweight 31 and acting in theopposite direction. In this case the force transfer system 23 can beconstructed as a lever 13 on or along which the counterweight is adaptedto be shifted. The force exerted by the counterweight 31 andcounteracting the force exerted by the movable member 9 is in this casedepending on the position of the counterweight in relation to themomentum point of the lever 13. Also in this case there may be provideda scale indicating different flow velocities and/or overpressures fordifferent positions of the counterweight on the lever.

As shown in FIGS. 5 and 6 the force transfer system may also be designedas a direct connection between the valve disk 14 and the movable member9, in which case the forces depending on the pressure prevailing in thenarcosis system and acting on respectively the valve disk and themovable member are directed in opposite directions substantially along acommon axis. This connection may comprise rigid force transfer members32 as shown in FIG. 5, or pliable members 33 as shown in FIG. 6, thelatter type of connection being exemplified by cords, belts, chains orresilient straps. Care should be taken in this connection to preventsuch shifting of the movable member as will appear upon smalloverpressures in the system (excess gas supply, expiration) fromexerting any closing action on the valve disk, there being a degree oflost motion causing such force to act on the valve disk 14 only athigher pressures prevailing in the system (compression of the bag). Whenrigid connecting member are used some kind of a telescope connection 34or other connections comprising lost motion should be used to absorb anyshifting movement of the movable member 9 due to small overpressures inthe system and to permit opening of the blow-off valve 8 to dischargecontinuous excess gas supply and expiration air without actuation of thevalve disk by the movable member in a closing direction.

As shown in FIG. 7 the force transfer may also be performed in such away that the movable member is caused to act directly against the valvedisk 14 of the blow-off valve 8 or a contact surface thereon. Accordingto FIG. 7 a bellows or movable piston or some other expandible member 9is connected to a first connection of a T-piece 35 which is stationarywith respect to the blow-off valve 8. The bag 3 is secured to the secondconnection of the T-piece 35, the third connection of which communicateswith the system via a branch conduit 36. By expansion of the bellows orshifting movement of the piston due to compression of the bag 3 the saidmember 9 will come into direct contact with the valve disk 14 to closethe blow-off valve 8. Care should be taken to prevent such contact fromtaking place at low overpressures in the system, for example byutilizing one or more magnets 21 attracting an iron part 22 of theexpandable member 9. As in the above described embodiments it isadvantageous to make use of a oneway valve 28 disposed between theblow-off valve 8 and the branch conduit 36 and a throttling means 29, 30upstream of the blow-off valve.

By providing the valve device according to the invention with a manuallyoperated deblocator 37 (FIGS. 2a and 2):) such as a push-button whichcan be locked in two positions it is possible to obtain a de-blocking ofthe blow-off valve in case a state of blocking has accidentallydeveloped due to faulty operation or adjustment. In the first position,which is indicated in continuous lines in FIGS. 20 and 2b, thepush-button 37 does not interfere with the force transfer system, but inthe second position, indicated by the dotted lines in FIG. 2b, thepush-button interferes with the force transfer system so as to preventthe force developed by the movable member from being transferred to theblow-off valve. The valve arrangement may also comprise means making itpossible manually to cause blocking or lock out of the blow-off valvewhich can be desired during certain types of narcosis, in particularwhen small amounts of gas are supplied not requiring any communicationbetween the system and the surrounding atmosphere.

With the valve construction as described above it is thus possible toeliminate the draw-backs of previously known valve constructions,especially those utilized in so-called semi-closed re-breathing systems.When utilizing the valve arrangement according to the invention there isobtained an anaesthetic gas supply system which on the one hand, is incommunication with the atmosphere at relatively small overpressures(high enough to overcome the biasing force of the blow-off valve) in thesystem, thus enabling expiration and discharge of excess gas underfavorable conditions and,

on the other hand, is shut off from communication with the atmosphere athigher overpressures created in the system by compression of thebreathing bag whereby control of the amount of gas thus introduced intothe respiratory ducts of the patient is facilitated and the loss of gasis minimized. By adjustment of the various adjustable pre-loadingdevices used in connection with the invention, such as a shiftablecounterweight or a magnet, the narcosis system in a convenient way maybe adjusted for use under varying conditions of continuous gas supply tothe system so that the valve always will open at a suitable selectedpressure, for example of an order of magnitude of 0.5 1.0 cm H 0.

By suitable dimensioning the movable member, suitably selecting theforce transfer system and the preloading thereof it is possible toensure that when compressing the bag the amount of gas which isdischarged from the system before the valve disk closes the valve due tothe action from the movable member will be substantially zero or amountto an arbitrary value.

Although the invention has been described with reference to are-breathing system for supporting natural breathing or administratingartificial respiration the valve arrangement according to the inventioncan be utilized in connection with all types or pressure controlled gascirculation systems in which it is desired to open or close a connectionto the atmosphere or to another system depending on the overpressureprevailing in the system.

The valve arrangement can be modified in many way without departing fromthe scope of the invention. For example, the force transfer from themovable member to the blow-off valve must not be obtained by the aboveexamplified arrangements, but any conventional mechanical, hydraulic,pneumatic or electrical force transfer system can be utlilized in placethereof. Although it has been preferred to arrange the movable memberand the blow-off valve adjacent to each other in one compact unit, theinvention is not limited to such an arrangement, but the valvearrangement according to the invention is capable of producing asufficient closing action irrespective of the location of the movablemember with respect to the blow-off valve and of the succession of saidmembers in relation to the gas flow direction.

What is claimed is:

l. A blow-off valve arrangement for use in a gas circulation systemventing to its environment in dependence on the internal pressureprevailing in the system, comprising a valve housing provided with avalve seat, a valve closing member, disposed on said seat from the sideof the environment and arranged to be biased in a closing positionagainst said seat so long as said internal pressure is lower than afirst predetermined overpressure value and to pass into an open positionunder the influence of said internal pressure when said internalpressure exceeds said first predetermined value; a movable memberexposed to said internal pressure and force transfer means disposedbetween said movable member and said valve closing member and arrangedto mechanically act to close said valve closing member when saidinternal pressure exceeds a second predetermined overpressure valuewhich is higher than said first predetermined overpressure, whereby saidgas circulation system vents to its environment for releasing surplusgas only when its internal pressure equals such an overpressure whosevalue lies between said two predetermined values.

2. A blow-off valve arrangement according to claim 1, wherein said forcetransfer means includes means exerting a force opposing the closing ofsaid valve closing member, said opposing force being greater than saidfirst predetermined value, but smaller than said second predeterminedvalue.

3. A blow-off valve arrangement according to claim 9 wherein said forcetransfer means includes a magnet which is adapted to attract an armaturedisposed on said lever of said force transfer means.

4. A. blow-off valve arrangement according to claim 3, wherein themagnet is disposed on the opposite side of the load arm of the lever,and wherein the armature is provided on the load arm.

5. A blow-off valve arrangement according to claim 4, wherein the magnetis disposed on the same side of the force arm of the lever as themovable member, and wherein the armature is provided on the force arm.

6. A blow-off valve arrangement according to claim 3 wherein thedistance between said magnet and said armature is adjustable.

7. A blow-off valve arrangement according to claim 9 wherein said forcetransfer means includes a counterweight provided on said load arm ofsaid double armed lever.

8. A blow-off valve arrangement according to claim 7, wherein saidcounterweight is slidable on the load arm of the lever.

9. A blow-off valve arrangement according to claim 29 wherein said forcetransfer means includes a doublearmed lever, the force arm of which isadapted to be acted upon by said movable member and the load arm ofwhich is adapted to act upon said valve closing member.

10. A blow-off valve arrangement according to claim 1 wherein the forcetransfer means comprises a pliable member such as a cord, belt, chain ora resilient strap interconnecting the movable member and the valveclosing element, the length of said pliable member exceeding theshortest distance between its fixing points to such an extent thatdisplacement of said movable member has effect on said valve closingmember only after said internal pressure has reached said secondpredetermined value.

1 l. A blow-off valve arrangement according to claim 1 wherein the forcetransfer means comprises a rigid force transfer member interconnectingthe movable member and the valve closing element, said rigid memberhaving means for producing lost motion to such an extent, thatdisplacement of said movable member has effect on said valve closingelement only after said internal pressure has reached said secondpredetermined value.

12. A blow-off valve arrangement according to claim 11, said lost motionis caused by a telescope connection means.

13. A blow-off valve arrangement according to claim 1 wherein themovable member is a bellows capable of expansion and contraction.

14. A blow-off valve arrangement according to claim 1, wherein themovable member is a piston which is slidable in a cylinder communicatingwith the gas system.

15. A blow-off valve arrangement for use in a rebreathing system forsupplying treating gas to a patient, which system includes are-circulation conduit having a first connection leading to the patient,a second connection for continuously supplying treating gas to thesystem, a third connection for a compressible bladder, and a one-wayvalve forcing the gas to flow in a predetermined direction in saidconduit, said arrangement comprising a valve housing adapted to beplaced directly in the circuit of said re-breathing system andcomprising: a valve seat in said valve housing, a cooperating valveclosing member disposed on said seat from the side of the environmentand arranged to be biased in a closing position against said seat solong as the internal pressure prevailing in the system is lower than afirst predetermined overpressure value and to pass into an open positionunder the influence of said internal pressure when said internalpressure exceeds said first predetermined value; a movable memberexposed to said internal pressure; and force transfer means disposedbetween said movable member and said valve closing member and arrangedto mechanically act to close said valve closing member when saidinternal pressure exceeds a second predetermined overpressure valuewhich is higher than said first predetermined overpressure, whereby saidgas circulation system vents to its environment for releasing surplusgas only when its internal pressure equals such an overpressure whosevalue lies between said two predetermined values.

16. A blow-off valve arrangement as defined in claim 15, wherein themovable member is disposed opposite said valve seat.

17. A blow-off valve arrangement as defined in claim 16, wherein thebladder is connected to the recirculation conduit at a location oppositesaid valve seat.

l8v A blow-off valve arrangement as defined in claim 17, wherein saidone-way valve is provided in the recir culation conduit at a locationbetween said valve seat and said bladder, said one-way valve beingadapted to prevent the gas-expelled from the bladder from flowing theshortest way to the valve closing element.

19. A blow-off valve arrangement as defined in claim 18, wherein thebladder and the movable member are disposed near the valve closingelement, thereby causing the gas expelled from the bladder uponcompression thereof to flow through almost the entire recirculationconduit before reaching the valve closing element.

20. A blow-off valve arrangement as defined in claim 18 furthercomprising a throttling means provided in the recirculation conduitbetween the patient connection and the valve closing element, saidthrottling means serving to delay the transfer from the bladder to thevalve closing element of the pressure surge caused by compression of thebladder.

21. A blow-off valve arrangement as defined in claim 20, wherein saidthrottling means comprises a disk provided with a central opening havingreduced cross-section.

22. A blow-off valve arrangement as defined in claim 20, wherein saidthrottling means comprises a jet or nozzle.

23. A valve unit for use in a re-breathing system for supplying treatinggas to a patient, said unit comprising: a valve housing having an inletconnection; an outlet connection; a ladder co nection; a blow-off valvecomprising a v ve seat an a cooperating valve closing element arrangedto keep the blow-off valve closed whenever the pressure acting on theclosing element from the interior of the valve housing is lower than afirst overpressure value; a connection to a movable member which iscapable of expansion and contraction in response to pressure variationswithin the valve housing; a double armed lever including a force arm anda load arm the force arm of which is adapted to be acted-upon by theforce produced by expansion of the movable member and the load arm ofwhich is adapted to act on said closing element to close the blow-offvalve only when the pressure in the valve housing exceeds a secondpressure value higher than said first value; a one-way valve dividingthe valve housing into two separate chambers for allowing gas to flowonly from a first of said chambers, with which the blow-off valvecommunicates, to the second chamber, into which the bladder connectionand the connection to the movable member opens.

24. A valve unit according to claim 30, wherein the load arm of saidlever is provided with an iron part adapted to be attracted by a magnetdisposed on the opposite side of the load arm with respect to theblowoff valve, the attraction force between the magnet and the iron partbeing such that the load arm does not act upon the closing until theoverpressure in the valve housing is higher than said second value.

25. A valve unit according to claim 30, further comprising a throttlingmeans disposed in the valve housing between the inlet connection and theblow-off valve.

26. A valve unit according to claim 25, wherein said throttling means isa partition having opening of reduced cross-section.

27. A blow-off valve arrangement as defined in claim 1 furthercomprising means disposed in said valve housing between said valve seatand said movable member for preventing any appreciable flow of gas in adirection from said member toward said valve seat within said valvehousing.

28. A blow-off valve arrangement as defined in claim 1 wherein saidforce transfer means are constituted by a portion of said movable memberand is disposed adjacent said valve closing member so that as thepressure in the gas circulation system increases the force transfermeans directly applies the force acting on said surface to said valveclosing member.

29. A blow-off valve arrangement according to claim 1, wherein saidmovable member is disposed on said housing at the side of said valveseat and connected with said valve closing member through force transfermeans in the form of a lever.

30. A blow-off valve arrangement according to claim 15, furthercomprising an absorber for removing pollutions such as carbon dioxide.

1. A blow-off valve arrangement for use in a gas circulation system venting to its environment in dependence on the internal pressure prevailing in the system, comprising a valve housing provided with a valve seat, a valve closing member, disposed on said seat from the side of the environment and arranged to be biased in a closing position against said seat so long as said internal pressure is lower than a first predetermined overpressure value and to pass into an open position under the influence of said internal pressure when said internal pressure exceeds said first predetermined value; a movable member exposed to said internal pressure and force transfer means disposed between said movable member and said valve closing member and arranged to mechanically act to close said valve closing member when said internal pressure exceeds a second predetermined overpressure value which is higher than said first predetermined overpressure, whereby said gas circulation system vents to its environment for releasing surplus gas only when its internal pressure equals such an overpressuRe whose value lies between said two predetermined values.
 2. A blow-off valve arrangement according to claim 1, wherein said force transfer means includes means exerting a force opposing the closing of said valve closing member, said opposing force being greater than said first predetermined value, but smaller than said second predetermined value.
 3. A blow-off valve arrangement according to claim 9 wherein said force transfer means includes a magnet which is adapted to attract an armature disposed on said lever of said force transfer means.
 4. A blow-off valve arrangement according to claim 3, wherein the magnet is disposed on the opposite side of the load arm of the lever, and wherein the armature is provided on the load arm.
 5. A blow-off valve arrangement according to claim 4, wherein the magnet is disposed on the same side of the force arm of the lever as the movable member, and wherein the armature is provided on the force arm.
 6. A blow-off valve arrangement according to claim 3 wherein the distance between said magnet and said armature is adjustable.
 7. A blow-off valve arrangement according to claim 9 wherein said force transfer means includes a counterweight provided on said load arm of said double armed lever.
 8. A blow-off valve arrangement according to claim 7, wherein said counterweight is slidable on the load arm of the lever.
 9. A blow-off valve arrangement according to claim 29 wherein said force transfer means includes a double-armed lever, the force arm of which is adapted to be acted upon by said movable member and the load arm of which is adapted to act upon said valve closing member.
 10. A blow-off valve arrangement according to claim 1 wherein the force transfer means comprises a pliable member such as a cord, belt, chain or a resilient strap interconnecting the movable member and the valve closing element, the length of said pliable member exceeding the shortest distance between its fixing points to such an extent that displacement of said movable member has effect on said valve closing member only after said internal pressure has reached said second predetermined value.
 11. A blow-off valve arrangement according to claim 1 wherein the force transfer means comprises a rigid force transfer member interconnecting the movable member and the valve closing element, said rigid member having means for producing lost motion to such an extent, that displacement of said movable member has effect on said valve closing element only after said internal pressure has reached said second predetermined value.
 12. A blow-off valve arrangement according to claim 11, said lost motion is caused by a telescope connection means.
 13. A blow-off valve arrangement according to claim 1 wherein the movable member is a bellows capable of expansion and contraction.
 14. A blow-off valve arrangement according to claim 1, wherein the movable member is a piston which is slidable in a cylinder communicating with the gas system.
 15. A blow-off valve arrangement for use in a re-breathing system for supplying treating gas to a patient, which system includes a re-circulation conduit having a first connection leading to the patient, a second connection for continuously supplying treating gas to the system, a third connection for a compressible bladder, and a one-way valve forcing the gas to flow in a predetermined direction in said conduit, said arrangement comprising a valve housing adapted to be placed directly in the circuit of said re-breathing system and comprising: a valve seat in said valve housing, a cooperating valve closing member disposed on said seat from the side of the environment and arranged to be biased in a closing position against said seat so long as the internal pressure prevailing in the system is lower than a first predetermined overpressure value and to pass into an open position under the influence of said internal pressure when said internal pressure exceeds said first predetermined value; a movable meMber exposed to said internal pressure; and force transfer means disposed between said movable member and said valve closing member and arranged to mechanically act to close said valve closing member when said internal pressure exceeds a second predetermined overpressure value which is higher than said first predetermined overpressure, whereby said gas circulation system vents to its environment for releasing surplus gas only when its internal pressure equals such an overpressure whose value lies between said two predetermined values.
 16. A blow-off valve arrangement as defined in claim 15, wherein the movable member is disposed opposite said valve seat.
 17. A blow-off valve arrangement as defined in claim 16, wherein the bladder is connected to the recirculation conduit at a location opposite said valve seat.
 18. A blow-off valve arrangement as defined in claim 17, wherein said one-way valve is provided in the recirculation conduit at a location between said valve seat and said bladder, said one-way valve being adapted to prevent the gas-expelled from the bladder from flowing the shortest way to the valve closing element.
 19. A blow-off valve arrangement as defined in claim 18, wherein the bladder and the movable member are disposed near the valve closing element, thereby causing the gas expelled from the bladder upon compression thereof to flow through almost the entire recirculation conduit before reaching the valve closing element.
 20. A blow-off valve arrangement as defined in claim 18 further comprising a throttling means provided in the recirculation conduit between the patient connection and the valve closing element, said throttling means serving to delay the transfer from the bladder to the valve closing element of the pressure surge caused by compression of the bladder.
 21. A blow-off valve arrangement as defined in claim 20, wherein said throttling means comprises a disk provided with a central opening having reduced cross-section.
 22. A blow-off valve arrangement as defined in claim 20, wherein said throttling means comprises a jet or nozzle.
 23. A valve unit for use in a re-breathing system for supplying treating gas to a patient, said unit comprising: a valve housing having an inlet connection; an outlet connection; a bladder connection; a blow-off valve comprising a valve seat and a cooperating valve closing element arranged to keep the blow-off valve closed whenever the pressure acting on the closing element from the interior of the valve housing is lower than a first overpressure value; a connection to a movable member which is capable of expansion and contraction in response to pressure variations within the valve housing; a double armed lever including a force arm and a load arm the force arm of which is adapted to be acted-upon by the force produced by expansion of the movable member and the load arm of which is adapted to act on said closing element to close the blow-off valve only when the pressure in the valve housing exceeds a second pressure value higher than said first value; a one-way valve dividing the valve housing into two separate chambers for allowing gas to flow only from a first of said chambers, with which the blow-off valve communicates, to the second chamber, into which the bladder connection and the connection to the movable member opens.
 24. A valve unit according to claim 30, wherein the load arm of said lever is provided with an iron part adapted to be attracted by a magnet disposed on the opposite side of the load arm with respect to the blow-off valve, the attraction force between the magnet and the iron part being such that the load arm does not act upon the closing until the overpressure in the valve housing is higher than said second value.
 25. A valve unit according to claim 30, further comprising a throttling means disposed in the valve housing between the inlet connection and the blow-off valve.
 26. A valve unit according to claim 25, wherein said throttling means is a partition having Opening of reduced cross-section.
 27. A blow-off valve arrangement as defined in claim 1 further comprising means disposed in said valve housing between said valve seat and said movable member for preventing any appreciable flow of gas in a direction from said member toward said valve seat within said valve housing.
 28. A blow-off valve arrangement as defined in claim 1 wherein said force transfer means are constituted by a portion of said movable member and is disposed adjacent said valve closing member so that as the pressure in the gas circulation system increases the force transfer means directly applies the force acting on said surface to said valve closing member.
 29. A blow-off valve arrangement according to claim 1, wherein said movable member is disposed on said housing at the side of said valve seat and connected with said valve closing member through force transfer means in the form of a lever.
 30. A blow-off valve arrangement according to claim 15, further comprising an absorber for removing pollutions such as carbon dioxide. 